Phosphatases remove phosphate groups from molecules previously activated by kinases and control most cellular signaling events that regulate cell growth and differentiation, cell-to-cell contacts, the cell cycle and oncogenesis. Protein phosphorylation is the ubiquitous strategy used to control the activities of eukaryotic cells. It is estimated that more than 1000 of the 10,000 proteins active in a typical mammalian cell are phosphorylated. The high energy phosphate which confers activation is transferred from adenosine triphosphate molecules to a protein by protein kinases, and is subsequently removed from the protein by protein phosphatases.
There appear to be three, evolutionarily-distinct protein phosphatase gene families (Carbonneau H. and Tonks N. K. (1992) Annu. Rev. Cell Biol. 8:463-93). They are the protein phosphatases (PPs) also known as serine/threonine phosphatases, the protein tyrosine phosphatases (PTPs), and the acid/alkaline phosphatases (APs).
PPs may be cytosolic or associated with a receptor and can be separated into four distinct groups. PP-IIC is a relatively minor phosphatase that is unrelated to the other three. The three principle PPs are composed of a homologous catalytic subunit coupled with one or more regulatory subunits. PP-I dephosphorylates many of the proteins phosphorylated by cylic AMP-dependent protein kinase and is therefore an important regulator of many cyclic AMP mediated, hormone responses in cells. PP-IIA has broad specificity for control of cell cycle, growth and proliferation, and DNA replication, and is the main phosphatase responsible for reversing the phosphorylations of serine/threonine kinases. PP-IIB, or calcineurin (Cn), is a Ca.sup.+2 activated phosphatase and is involved in the regulation of such diverse cellular functions as ion channel regulation, neuronal transmission, gene transcription, muscle glycogen metabolism, and lymphocyte activation. Cn is found in all tissues but is particularly abundant in the brain. Cn also deactivates PP-I by dephosphorylation of a specific protein inhibitor of PP-I (P. Cohen (1989) Annu. Rev. Biochem. 58:453-508). Thus Cn has the potential to indirectly regulate many cyclic-AMP mediated cell functions as well.
Cn is composed of a catalytic A subunit (CnA) and a regulatory B subunit (CnB). Cn is activated synergistically by the binding of Ca.sup.+2 /calmodulin to CnA, and the binding of Ca.sup.+2 to CnB. CnB is characterized by four Ca.sup.+2 binding domains distributed over the length of the molecule, and by an N-terminal myristic acid blocking group (Aitken, A. et al. (1984) Eur. J. Biochem. 139:663-71). The myristoyl group may play a role in the interaction of the A and B subunits or in membrane interactions. Multiple isozymes of CnB exist in mammals.
The regulation of PPs, and particularly of Cn, has implications for the control of a variety of disease conditions. The immunosuppressive agents cyclosporin and FK506 appear to act in part by inhibiting Cn mediated T-cell activation, indicating the importance of Cn in the immune response (Schwaninger M. Et al. (1993) J. Biol Chem. 268:23111-15). Cn, as well as other PPs, appears to be important for synaptic transmission in the brain and may be involved in learning and memory disorders (Mulkey R. M. et al. (1993) Science 261:1051-55). The role of PPs in cell cycle regulation, cell proliferation, and gene transcription indicate that they may also be important in the control of cancer.
The discovery of polynucleotides encoding protein phosphatases, and the molecules themselves, provides a means to investigate the role of these molecules in the wide array of cellular functions controlled by protein phosphorylation. Discovery of molecules related to protein phosphatases satisfies a need in the art by providing new diagnostic or therapeutic compositions useful in immunological and neurological disorders and cancer.